Controlled nanopore formation in graphene/graphene oxide nanosheets: implication for water transport

Two-dimensional (2D) sheets of graphene/graphene oxide are the building blocks of a wide range of material architectures with strong application potential in energy storage and harvesting, and environmental remediation. A consistent issue with continuous 2D sheets, especially when hundreds of such 2D sheets are stacked tightly to form films and electrodes, is their low mass transport characteristics through the assembled structure. To overcome this problem, we report a sequential, two-step photochemical technique comprising nucleation of defects on 2D nanosheets of graphene/graphene oxide by long-wavelength (UVA/UVB) irradiation, followed by the growth of nanopores in H2O2-based etching triggered by short-wavelength (UVC) irradiation. We demonstrate our ability to tailor the size (10-100 nm) and level of porosity (16-60%) in holey graphene oxide (h-GO). To test the holey GO we synthesized, we produced the nanofiltration membranes using h-GO with different pore sizes. Membranes made from hGO nanosheets with 60 nm pores exhibited up to a 3.7-fold increase in water permeance and an 10% increase in selectivity compared to those produced by pristine GO. We attribute this unusual behavior to the presence of water transport highways (the nanopores) and a smaller interlayer distance of the hGO sheets arising from a complex balance in hydroxylation and deoxygenation reactions during the photochemical process. We demonstrated successful transition of the method to a flow-based synthesis approach with highly enhanced production rates (188 mg/h, an about 30-fold increase over the batch process), thereby accelerating sustainable and automated manufacturing of perforated graphene materials and their adoption in industrial uses.